I recently came back from London. I went through the following process:
1. family or photography trip? Family for me, so I wanted something with auto-everything mode for family shots--this eliminated the Leicas and Nikons.
2. size/weight: if photo trip, image quality is priority; otherwise, size/weight/comfort/portability. Latter for me, as this was family trip. This eliminated the full frame (and DX) DSLRs.
3. Lens range? I prefer UW zoom for travel, and fast wides for interior low-light.
4. video: if desired, mirrorless please. Nikon's implementation is complicated, requiring a switch and button to get video (Leica M240 is even less effective due to the nature of the camera).
For London, I started Nikon/Leica M combo but finalized with m43: 2 bodies (EM1 and EP5), with the Oly 12/17 primes and the Pany 7-14/35-100 zooms (used mostly 7-14 and 17, one on each body). Small bag, easy to manage in airport/plane/etc. Stuff fit in my coat pockets (EP5/17) or small bag. Full frame equivalents would have meant Domke F2 or similar; Leica M would not work b/c no full auto mode (as much as I used it in my past DC trip and produced beautiful images). Hope this helps.